This invention relates to the regeneration of solder stripping solutions and, more particularly, to the regeneration of ammonium bifluoride-hydrogen peroxide solutions used to strip solder from copper plated printed circuit boards.
Printed circuit boards are well-known and are used extensively in electrical equipment. Basically, a flat sheet of dielectric material such as epoxy, polyester, or other resin, contains the desired circuit design imprinted on its surface in the form of copper conductor circuits. Some circuit boards contain contact tabs or fingers to which connections between the circuit and other circuits or circuit boards are made.
Solders, which are essentially tin-lead binary alloys, are used for a number of purposes in printed circuit board manufacture. The tin-lead binary alloy may contain, by weight, about 1-99% tin, balance lead and a typical alloy contains about 60% tin-40% lead. Incidental impurities, as well as other alloying elements, may also be present in the solder. Basically, the solder is used as a protective covering (solder mask) for the bare copper circuit (SMOBC) type boards and for boards containing contact tabs and the solder must be stripped from the boards to expose the copper surfaces.
Different fluorine containing compositions may be used to strip solder from printed circuit boards and for convenience the following description of the invention will relate principally to the composition most used by industry, to wit, an aqueous solution of ammonium bifluoride and hydrogen peroxide. Peroxide stabilizers are commonly used in the stripper composition to inhibit decomposition of the peroxide and release of oxygen and to prevent severe attack on the copper and board. U.S. Pat. Nos. 3,841,905 and 3,926,699 disclose these type compositions and a method for using the compositions to strip lead-tin solder from a layer of copper on circuit boards. U.S. Pat. No. 4,397,753 discloses a different solder stripping solution containing a hydroxyphenol in an aqueous solution of a nitro-substituted aromatic compound, an inorganic acid such as fluoboric acid and thiourea. The disclosures of the above patents are herein incorporated by reference.
In general, tin-lead binary alloy deposits on the printed circuit boards are removed by immersing the boards in the stripper solution. The contacting may also be effected by spraying. To facilitate the stripping of the solder from those boards having contact tabs, those portions of the printed circuit board which are immediately adjacent and contiguous with the inner-most end portions of the circuit board are blocked or masked off with a resist continuous layer which is resistant to and unaffected by the aqueous stripper solution. This masking layer should be one that is readily removable manually or otherwise, and is exemplified by Scotch tape or an electricians dielectric tape.
During the stripping operation, the solution becomes contaminated with the lead and tin of the solder, copper from the circuit board and resultant complex chemical reaction products. A sludge containing such contaminants also forms. Eventually, the solution is depleted or spent and must be discarded or, preferably, regenerated for reuse. Waste treatment of spent chemical solutions is a serious problem and industry has long been aware that recovery of the exhausted solution would be of substantial economic benefit, especially if the recovered product performs like the original composition.
In general, the ammonium bifluoride-hydrogen peroxide solutions are now replenished during use by addition of hydrogen peroxide to the solution when the concentration falls below set limits. Replenishment of the ammonium bifluoride is ordinarily not necessary. The solutions cannot be replenished in any event, however, when the impurities, such as tin, lead and copper, exceed certain levels and it is necessary at this point to remove these materials from the solution followed by, if needed, replenishment of the active ingredients.
Regeneration of spent solder stripping solutions is very difficult however, because not only must certain impurities be effectively removed from solutions containing a variety of impurities, including unknown complex reaction products, but the solution should not be significantly changed in pH or volume which would require further regeneration procedures. For example, precipitation of tin as a hydroxide may be effected by adding an excess of hydroxyl ions to the solution. Unfortunately, the solution volume increase may be significant and an acid would then have to be added to lower the pH to the desired range. Other separation techniques such as evaporation have also proved ineffective.
Tin is perhaps the most troublesome impurity in terms of removal from the spent solder stripping solution and it is an object of this invention to provide an efficient method for the tin removal.
Another object of the present invention is to provide a method for the regeneration for reuse of spent solder stripping solutions.